U.S. patent number 8,483,571 [Application Number 12/828,278] was granted by the patent office on 2013-07-09 for optical beam splitter for use in an optoelectronic module, and a method for performing optical beam splitting in an optoelectronic module.
This patent grant is currently assigned to Avago Technologies General IP (Singapore) Pte. Ltd.. The grantee listed for this patent is Laurence R. McColloch, Pengyue Wen. Invention is credited to Laurence R. McColloch, Pengyue Wen.
United States Patent |
8,483,571 |
McColloch , et al. |
July 9, 2013 |
Optical beam splitter for use in an optoelectronic module, and a
method for performing optical beam splitting in an optoelectronic
module
Abstract
An optical beam splitter for use in an optoelectronic module and
method are provided. The optical beam splitter is configured to
split a main beam produced by a laser into at least first and
second light portions that have different optical power levels. The
first light portion, which is to be coupled into an end of a
transmit optical fiber of an optical communications link, has an
optical power level that is within eye safety limits and yet has
sufficient optical power to avoid signal degradation problems. The
optical power level of the first light portion is less than the
optical power level of the second light portion. The optical beam
splitter is capable of being implemented in a unidirectional or a
bidirectional optical link.
Inventors: |
McColloch; Laurence R. (Santa
Clara, CA), Wen; Pengyue (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
McColloch; Laurence R.
Wen; Pengyue |
Santa Clara
San Jose |
CA
CA |
US
US |
|
|
Assignee: |
Avago Technologies General IP
(Singapore) Pte. Ltd. (Singapore, SG)
|
Family
ID: |
44485300 |
Appl.
No.: |
12/828,278 |
Filed: |
June 30, 2010 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20120002284 A1 |
Jan 5, 2012 |
|
Current U.S.
Class: |
398/141; 398/200;
359/629; 398/214 |
Current CPC
Class: |
G02B
6/4214 (20130101); G02B 6/4206 (20130101); H01S
5/02325 (20210101); H01S 5/0683 (20130101); H01S
5/005 (20130101); G02B 6/4246 (20130101); H01S
5/02251 (20210101); Y10T 29/49899 (20150115); Y10T
29/4987 (20150115) |
Current International
Class: |
G02B
27/14 (20060101) |
Field of
Search: |
;359/558,566,569,572,629,636 ;385/15,31,37
;398/82,84,87,141,168-170,200,201,212,214 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2374142 |
|
Oct 2002 |
|
GB |
|
2004-212847 |
|
Jul 2004 |
|
JP |
|
WO 00/13051 |
|
Mar 2000 |
|
WO |
|
Primary Examiner: Allen; Stephone
Assistant Examiner: Kakalec; Kimberly N
Claims
What is claimed is:
1. An optical beam splitter for use in an optoelectronic module,
the optical beam splitter comprising: a substrate having at least
upper and lower surfaces that are substantially parallel to one
another; at least a first diffractive optical element formed on the
outside of the lower surface of the substrate, the first
diffractive optical element receiving a main beam of light produced
by a light source and splitting the main beam into at least first
and second light portions L1 and L2, respectively, and tilting at
least the first light portion L1 at a first preselected non-zero
tilt angle relative to an imaginary line that is normal to the
lower surface of the substrate and that extends between the lower
and upper surfaces of the substrate, the first and second light
portions L1 and L2 each having a particular percentage of an
optical power of the main light beam, and wherein the second light
portion L2 has a substantially larger percentage of the optical
power of the main light beam than the first portion L1; and at
least a first refractive optical element formed on the outside of
the upper surface of the substrate, the first refractive optical
element receiving the tilted first light portion L1 and directing
the first light portion L1 onto a lens assembly of the
optoelectronic module to optically couple the first light portion
L1 into an end of an optical fiber coupled to the lens assembly,
and wherein the optical power of the first light portion L1 that is
directed onto the lens assembly is within a human eye safety
limit.
2. The optical beam splitter of claim 1, wherein the first light
portion L1 has about 20% of the optical power of the main light
beam and wherein the second light portion L2 has about 80% of the
optical power of the main light beam.
3. The optical beam splitter of claim 2, wherein the first
diffractive element directs the second light portion L2 onto an
optical beam stop.
4. The optical beam splitter of claim 1, wherein the first
diffractive optical element splits the main light beam into the
first and second light portions L1 and L2 and into a third light
portion L3, and wherein the optical beam splitter further
comprises: at least a first mirroring optical element formed on the
upper surface of the substrate, and wherein the first mirroring
optical element receives the third light portion L3 and directs the
third light portion L3 through the bottom surface of the substrate
onto a monitor light detector of the optoelectronic module that is
used to monitor an optical output power level of the light source
of the optoelectronic module.
5. The optical beam splitter of claim 4, wherein at least the first
mirroring optical element and the first refractive optical element
are formed of replicated epoxy.
6. The optical beam splitter of claim 1, wherein light passing out
of an end of an optical fiber coupled to the lens assembly is
directed by the lens assembly onto the first refractive element,
and wherein the first refractive element tilts the received light
at a predetermined angle relative to an imaginary line that is
normal to the upper surface of the substrate and that extends
between the upper and lower surfaces of the substrate and directs
the tilted received light onto the first diffractive optical
element, and wherein the first diffractive optical element directs
the tilted received light toward to the upper surface of the
substrate, and wherein the optical beam splitter further comprises:
at least a second mirroring optical element formed on the upper
surface of the substrate, and wherein the second mirroring optical
element reflects the light directed by the first diffractive
optical element toward the upper surface of the substrate toward
the lower surface of the substrate; and at least a second
refractive optical element formed on the lower surface of the
substrate, the second refractive optical element directing the
light reflected by the second mirroring optical element onto a
receiver light detector of the optoelectronic module disposed
adjacent the lower surface of the substrate.
7. The optical beam splitter of claim 6, wherein at least the first
and second refractive optical elements are formed of replicated
epoxy.
8. The optical beam splitter of claim 6, wherein the first
diffractive optical element and the second mirroring optical
element are formed of metal.
9. The optical beam splitter of claim 1, wherein the first
preselected tilt angle is between about 5 degrees and 10
degrees.
10. The optical beam splitter of claim 9, wherein the first
preselected tilt angle is about 8 degrees.
11. A method of performing optical beam splitting in an
optoelectronic module, the method comprising: with a first
diffractive optical element formed on the outside of a lower
surface of a substrate, receiving a main beam of light produced by
a light source of the optoelectronic module; with the first
diffractive optical element, splitting the main beam into at least
first and second light portions L1 and L2, respectively, and
tilting at least the first light portion L1 at a first preselected
non-zero tilt angle relative to an imaginary line that is normal to
the lower surface of the substrate and that extends between the
lower and upper surfaces of the substrate, wherein the first and
second light portions L1 and L2 each have a particular percentage
of an optical power of the main light beam, and wherein the second
light portion L2 has a substantially larger percentage of the
optical power of the main light beam than the first portion L1; and
with at least a first refractive optical element formed on the
outside of the upper surface of the substrate, receiving the tilted
first light portion L1 and directing the first light portion L1
onto a lens assembly of the optoelectronic module to optically
couple the first light portion L1 into an end of an optical fiber
coupled to the lens assembly, and wherein the optical power of the
first light portion that is directed by the first refractive
optical element onto the lens assembly is within a human eye safety
limit.
12. The method of claim 11, wherein the first diffractive element
directs the second light portion L2 onto an optical beam stop.
13. The method of claim 11, wherein the first diffractive optical
element splits the main light beam into the first and second light
portions L1 and L2 and into a third light portion L3, and wherein
the method further comprises: with a first mirroring optical
element formed on the upper surface of the substrate, receiving the
third light portion L3 and directing the third light portion L3
through the lower surface of the substrate onto a monitor light
detector of the optoelectronic module that is used to monitor an
optical output power level of the light source of the
optoelectronic module.
14. The method of claim 13, wherein at least the first mirroring
optical element and the first refractive optical element are formed
of replicated epoxy.
15. The method of claim 11, further comprising: with the first
refractive optical element, receiving light passing out of an end
of an optical fiber coupled to the lens assembly and directing the
received light onto the first diffractive optical element; with the
first diffractive element, receiving the light directed by the
first refractive element onto the first diffractive element and
directing the received light toward to the upper surface of the
substrate; with a second mirroring optical element formed on the
upper surface of the substrate, reflecting the light directed by
the first diffractive optical element toward the upper surface of
the substrate toward the lower surface of the substrate; and with a
second refractive optical element formed on the lower surface of
the substrate, directing the light reflected by the second
mirroring optical element onto a receiver light detector of the
optoelectronic module.
16. The method of claim 15, wherein at least the first and second
refractive optical elements are formed of replicated epoxy.
17. The method of claim 15, wherein the first diffractive optical
element and the second mirroring optical element are formed of
metal.
18. The method of claim 11, wherein the first preselected tilt
angle is between about 5 degrees and 10 degrees.
19. The method of claim 18, wherein the first preselected tilt
angle is about 8 degrees.
20. The method of claim 11, wherein the first light portion L1 has
about 20% of the optical power of the main light beam and wherein
the second light portion has about 80% of the optical power of the
main light beam.
Description
TECHNICAL FIELD OF THE INVENTION
The invention relates to optoelectronic modules. More particularly,
the invention relates to an optical beam splitter for use in an
optoelectronic module for splitting a beam into at least two
portions that have different levels of optical power.
BACKGROUND OF THE INVENTION
In optical communications networks, optoelectronic modules are used
to transmit and/or receive optical signals over optical fibers. The
optoelectronic module may be configured as an optical transmitter
that transmits optical signals, an optical receiver that receives
optical signals, or an optical transceiver that transmits and
receives optical signals. On the transmit side of an optical
transmitter or transceiver module, a light source (e.g., a laser
diode) generates amplitude modulated optical signals that represent
data, which are optically coupled by an optics system of the module
into an end of a transmit optical fiber. The signals are then
transmitted over the transmit fiber to a receiver node of the
network. On the receive side of an optical receiver or transceiver
module, an optics system of the module receives optical signals
output from an end of a receive optical fiber and focuses the
optical signals onto an optical detector (e.g., a photodiode),
which converts the optical energy into electrical energy.
In some laser-based optoelectronic modules, a portion of the light
that is produced by the laser is used to monitor the optical output
power level of the laser and to adjust the optical output power
level of the laser as needed. Typically, it is desirable to
maintain the optical output power level of the laser at a
substantially constant, predetermined level during operations. To
accomplish this, many optoelectronic modules include components
that together make up a feedback control system for monitoring the
average optical output power level of the laser and adjusting the
bias and/or modulation currents of the laser as needed to maintain
that average optical output power level at a substantially
constant, predetermined level. The feedback path components
typically include a beam splitter, a monitor photodiode,
analog-to-digital (ADC) circuitry, and controller circuitry. The
beam splitter causes a portion of the beam that is produced by the
laser to be split off and directed onto a monitor photodiode. The
monitor photodiode produces an analog electrical signal in response
to the light that is directed onto it by the beam splitter. The
analog electrical signal is converted into a digital electrical
signal by the ADC circuitry. The controller circuitry processes the
digital electrical signal and causes the laser modulation and/or
bias currents to be adjusted accordingly.
Beam splitters are manufactured in a variety of configurations and
typically comprise one or more reflective, refractive and/or
diffractive elements. In a typical beam splitter configuration, a
first portion of the main beam produced by the laser passes through
the beam splitter with very little if any of the light being
reflected, refracted or diffracted. This portion of the main beam
is then coupled into the end of the transmit optical fiber for
transmission over the transmit optical fiber. At the same time, a
second portion of the main beam is reflected, refracted and/or
diffracted by the beam splitter to cause the second portion to be
directed onto the monitor photodiode.
Usually, the first and second portions each contain about 50% of
the optical power that was contained in the main beam. This
symmetric, or even, split of the optical power can cause problems
in some cases. Typical lasers that are used in optoelectronic
modules produce light having optical power levels that are much
greater than safety limits for the human eye. Even at 50% of the
optical power of the main beam, the first portion of the light will
have an optical power level that is greater than eye safety limits.
It is generally not possible, or at least very difficult, to run a
laser at the high speed required for the optical communications
link and simultaneously reduce the optical output power level of
the laser to a level that is within eye safety limits. For this
reason, steps are often taken to ensure that the light that is to
be transmitted over the transmit optical fiber is attenuated to an
optical power level that is within the safety limits.
Accordingly, a need exists for an optical beam splitter for use in
an optoelectronic module that is capable of providing an uneven, or
asymmetrical, split of the main beam produced by the laser such
that the portion of the light that is split off and coupled into
the end of the transmit optical fiber as the optical data signal
has an optical power level that is within human eye safety limits
and yet that has sufficient optical power to avoid signal
degradation problems.
SUMMARY OF THE INVENTION
The invention is directed to an optical beam splitter for use in an
optoelectronic module and a method for optically splitting a beam
in an optoelectronic module. The optical beam splitter comprises a
substrate having at least upper and lower surfaces that are
parallel to one another, at least a first diffractive optical
element formed on the lower surface of the substrate, and at least
a first refractive optical element formed on the upper surface of
the substrate. The first diffractive optical element receives a
main beam of light produced by the light source and splits the main
beam into at least first and second light portions L1 and L2,
respectively. The first diffractive optical element tilts at least
the first light portion L1 at a first preselected tilt angle
relative to an imaginary line that is normal to the lower surface
of the substrate and that extends between the lower and upper
surfaces of the substrate. The first and second light portions L1
and L2 each have a particular percentage of the optical power of
the main light beam. The second light portion L2 has a
substantially larger percentage of the optical power of the main
light beam than the first portion L1. The first refractive optical
element receives the tilted first light portion L1 and directs the
first light portion L1 onto a lens assembly of the optoelectronic
module to optically couple the first light portion L1 into an end
of an optical fiber coupled to the lens assembly. The optical power
of the first light portion L1 that is directed onto the lens
assembly is within a human eye safety limit.
The method comprises the following. With a first diffractive
optical element formed on a lower surface of a substrate, receiving
a main beam of light produced by a light source of the
optoelectronic module. With the first diffractive optical element,
splitting the main beam into at least first and second light
portions L1 and L2, respectively, and tilting at least the first
light portion L1 at a first preselected tilt angle relative to an
imaginary line that is normal to the lower surface of the substrate
and that extends between the lower and upper surfaces of the
substrate. The first and second light portions L1 and L2 each have
a percentage of the optical power of the main light beam, and the
second light portion L2 has a substantially larger percentage of
the optical power of the main light beam than the first portion L1.
With a first refractive optical element formed on the upper surface
of the substrate, receiving the tilted first light portion L1 and
directing the first light portion L1 onto a lens assembly of the
optoelectronic module to optically couple the first light portion
L1 into an end of an optical fiber coupled to the lens assembly.
The optical power of the first light portion that is directed by
the first refractive optical element onto the lens assembly is
within a human eye safety limit.
These and other features and advantages of the invention will
become apparent from the following description, drawings and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts an illustrative embodiment of a bidirectional
optical link comprising an optical fiber and optoelectronic modules
coupled to opposite ends of an optical fiber that incorporate
optical beam splitters in accordance with an illustrative
embodiment of the invention.
FIG. 2 illustrates a side cross-sectional view of an illustrative
embodiment of one of the optical beam splitters illustrated in FIG.
1.
FIGS. 3A and 3B illustrate bottom and top perspective views,
respectively, of a glass wafer having arrays of the optical
elements of the optical beam splitter shown in FIG. 2 formed on the
bottom and top surfaces thereof.
FIGS. 4A and 4B illustrate the bottom and top surfaces of the wafer
shown in FIGS. 3A and 3B after an epoxy replication process has
been used to form the refractive optical elements on the bottom
surface of the wafer and to form the optical coupling structure on
the top surface of the wafer.
FIGS. 5A and 5B illustrate bottom and top perspective views,
respectively, of the optical beam splitter shown in FIG. 2.
FIG. 6 illustrates a top perspective view of an optical transceiver
module having an optical beam splitter holder coupled therewith
that holds the optical beam splitter shown in FIG. 2.
FIG. 7 illustrates a cross-sectional perspective view of the
optical beam splitter holder shown in FIG. 6 with the optical beam
splitter held therein.
FIG. 8 illustrates a perspective view of the optical transceiver
module shown in FIG. 6 having an optical connector coupled to it,
which, in turn, has the lens assembly shown in FIG. 2 coupled to
it.
FIGS. 9A and 9B illustrate top and bottom perspective views,
respectively, of the optical connector shown in FIG. 8 having the
lens assembly shown in FIG. 2 coupled to it.
FIG. 10 illustrates a cross-sectional side view of the optical
transceiver module shown in FIG. 8 coupled with the optical
connector shown in FIGS. 9A and 9B, which, in turn, is coupled with
the lens assembly.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
The invention is directed to an optical beam splitter for use in an
optoelectronic module. The optical beam splitter is configured to
split a main beam produced by a laser into at least first and
second light portions that have different optical power levels. The
first light portion, which is to be coupled into an end of a
transmit optical fiber of an optical communications link, has an
optical power level that is within eye safety limits and yet has
sufficient optical power to avoid signal degradation problems. The
first light portion has an optical power level that is less than
the optical power level of the second light portion. The
optoelectronic module in which the optical beam splitter is
employed may be an optical transmitter module or an optical
transceiver module. The optical communications link may be a
unidirectional optical link or a bidirectional optical link.
However, in order to demonstrate the various capabilities and
advantages of the optical beam splitter, a bidirectional
configuration of the optical beam splitter that enables it to be
used in a bidirectional optical link will be now described with
reference to the figures.
FIG. 1 depicts an illustrative embodiment of a bidirectional
optical link 1 comprising an optical fiber 10 and optoelectronic
modules 20 and 30 coupled to opposite ends of an optical fiber 10,
each of which incorporates an optical beam splitter in accordance
with an illustrative embodiment of the invention. In accordance
with this illustrative embodiment, the optoelectronic modules 20
and 30 are optical transceiver modules. The optical transceiver
module 20 coupled to end 10a of the optical fiber 10 includes a TX
20a, an RX 20b, and an optical coupling system 50. The optical
coupling system 50 of the optical transceiver module 20
incorporates an optical beam splitter 50a and one or more
refractive, reflective and/or diffractive optical elements (not
shown), as will be described below in detail with reference to FIG.
2. The optical beam splitter 50a and the other optical elements
(not shown) may be separate elements or they may be integrated into
a single integrated optical device, as will be described below with
reference to FIG. 2. The optical transceiver module 30 coupled to
end 10b of the optical fiber 10 includes a TX 30a, an RX 30b, and
an optical coupling system 60. Like the optical coupling system 50,
the optical coupling system 60 incorporates an optical beam
splitter 60a and one or more refractive, reflective and/or
diffractive optical elements. The optical transceiver module 20
includes a beam stop 70a and a monitor photodiode 70b. Likewise,
the optical transceiver module 30 includes a beam stop 80a and a
monitor photodiode 80b.
As will be described below in more detail with reference to FIG. 2,
the TXs 20a and 30a of the optical transceiver modules 20 and 30
each include at least one light source, which is typically a laser
diode, such as a vertical cavity surface emitting laser diode
(VCSEL), for example, and light source driver circuitry. The RXs
20b and 30b of the optical transceiver modules 20 and 30 each
include at least one optical-to-electrical (OE) conversion device,
such as a photodiode, and receiver circuitry. The optical fiber 10
is typically a multi-mode (MM) optical fiber, such as, for example,
an oM3 mM optical fiber. In accordance with this illustrative
embodiment, the TXs 20a and 30a and the RXs 20b and 30b all operate
at the same wavelength. The optical coupling systems 50 and 60
allow the optical transceiver modules 20 and 30 to simultaneously
transmit and receive optical data signals that are at the same
wavelength, which may be, for example, 850 nanometers (nm), 1310 nm
or 1550 nm. The invention, however, is not limited with respect to
the wavelength that is used for this purpose.
The optical transceiver modules 20 and 30 operate as follows. In
the transmit direction of the optical transceiver module 20, the TX
20a produces a main beam of laser light having wavelength .lamda.1,
which is received by the optical coupling system 50. The optical
coupling system 50 collimates the main beam, splits the main beam
into at least first and second light portions L1 and L2, and tilts
at least the first light portion L1 at a preselected angle relative
to the angle of incidence of the main beam on the optical coupling
system 50. In FIG. 1, the tilting of the first light portion L1 is
not shown for ease of illustration. An example of an actual
physical implementation of the optical coupling system 50 is
described below in detail with reference to FIG. 2.
With reference again to FIG. 1, the first light portion L1 has less
optical power than the second light portion L2. The first light
portion L1 has between about 10% and 30% of the optical power of
the main beam, and typically has about 20% of the optical power of
the main beam. The second light portion L2 has between about 70%
and 90% of the optical power of the main beam, and typically has
about 80% of the optical power of the main beam. As will be
described below in more detail with reference to FIG. 2, most of
the second light portion L2 is absorbed by the beam stop 70a and a
very small amount of the second light portion L2 is received by the
monitor photodiode 70b and used as optical feedback to control
optical output power level of the laser diode. Although the beam
stops/monitor photodiodes 70a/70b and 80a/80b are depicted in FIG.
1 as being collocated, this is merely for ease of illustration, as
will become clear from the description below of FIG. 2.
The first light portion L1 passes through the beam splitter 50a and
is brought to a focal point by the optical coupling system 50 at
the end 10a of the MM optical fiber 10. The first light portion L1
corresponds to the first optical data signal, which is then
transmitted along the MM optical fiber 10 to the optical
transceiver 30 coupled to the opposite end 10b of the MM optical
fiber 10. As indicated above, most of the second light portion L2
is directed by the optical coupling system 50 onto the beam stop
70a while a very small portion of the second light portion L2 is
directed onto the monitor photodiode 70b. Typically, the beam stop
70a receives and absorbs about 80% of the optical power of the main
beam. By absorbing the light received thereby, the beam stop 70a
prevents near-end stray light from affecting the performance of the
optical transceiver module 20. As indicated above, the monitor
photodiode 70b is used in a power monitoring feedback loop to
monitor the optical output power of the laser diode of the TX 20a.
The monitor photodiode 70b typically receives only about 1% to 2%
of the optical power of the main beam.
Transparent stubs 90a and 90b are placed on the ends 10a and 10b,
respectively, of the MM optical fiber 10. The stubs 90a and 90b
have first ends that attach to respective optical ports 105 and
115, respectively, of the optical transceiver modules 20 and 30,
respectively. The stubs 90a and 90b have second ends that attach to
the ends 10a and 10b, respectively, of the MM optical fiber 10. The
ends of the stubs 90a and 90b that attach to the optical ports 105
and 115 have anti-reflection coatings thereon. The ends of the
stubs 90a and 90b that have the anti-reflection coatings thereon
will be referred to hereinafter as the entrance facets of the stubs
90a and 90b. The ends of the stubs 90a and 90b that attach to the
ends 10a and 10b of the MM optical fiber 10 are attached in such a
way that no light is reflected at the interfaces of the stubs 90a
and 90b and the ends 10a and 10b, respectively, of the MM optical
fiber 10. The stubs 90a and 90b essentially eliminate all interface
reflections at the optical ports 105 and 115, thereby preventing
signal degradation due to cross-talk that might otherwise be caused
by reflections of light at these interfaces. It should also be
understood that as part of normal operation of the optical
transceiver modules 20 and 30, the fiber 10 may be attached to and
detached from the respective optical ports 105 and 115, and hence
to and from the respective stubs 90a and 90b. Stubs of the type
that may be used for this purpose are well known in the art.
Therefore, persons of ordinary skill in the art will understand, in
view of the description provided herein, the manner in which the
stubs 90a and 90b may be designed and implemented in the
bi-directional optical link 1 shown in FIG. 1.
In the receive direction of the optical transceiver module 20, an
optical data signal of wavelength .lamda.1 that has been
transmitted by the optical transceiver module 30 over the MM
optical fiber 10 is received by the optical coupling system 50. The
stub 90b prevents interface reflections from occurring in the
optical transceiver module 30 that might otherwise result in
cross-talk and signal degradation. The beam splitter 50a reflects
the received optical data signal in a direction toward the RX 20b
and the light is focusing by the optical coupling system to a focal
point on a photodiode (not shown) of the RX 20b. The light that is
focused on the photodiode of the RX 20b has about 70% to 90%, and
typically about 80%, of the optical power of the optical data
signal that passes out of the end 10a of the MM optical fiber 10
and into the optical coupling system 50. The other 10% to 30% of
the received light, and typically about 20%, may pass into the
laser diode (not shown) of the TX 20. This relatively small amount
of light is not focused on the laser diode, but is somewhat
diffuse, and typically will not significantly degrade the
performance of the laser diode of the TX 20.
The operations for the transmit and receive directions of the
optical transceiver module 30 are identical to the operations
described above for the transmit and receive directions of the
optical transceiver module 20. Therefore, in the interest of
brevity, the operations of the optical transceiver module 30 in the
transmit and receive directions will not be described herein. It
should also be noted that although the optical transceiver modules
20 and 30 shown in FIG. 1 are depicted as each having a single TX
channel and a single RX channel and associated components, the
invention may be implemented in a parallel optical transceiver
module that has a plurality of instances of the optical transceiver
modules 20 and 30 and multiple respective optical fibers 10 linking
the respective modules together.
The stubs 90a and 90b are optional. If the stubs 90a and 90b are
not used and no other techniques are used to eliminate interface
reflections, the link 1 may still operate satisfactorily, but there
may be a performance penalty in terms of increased signal
degradation. The interface reflection is typically only
approximately 4% of the total energy of the optical data signal.
Therefore, it is possible to have satisfactory performance without
totally eliminating interface reflection. Another option to using
the stubs 90a and 90b is to employ some form of electronic
equalization in the RXs 20b and 30b to cancel out the interface
reflection. It is possible to use electronic equalization for this
purpose due to the fact that any interface reflection generally
will always occur at the same instant in time relative to the
transmission of the optical data signal at the near-end optical
transceiver. For example, electronic equalizers (not shown) may be
used in the RXs 20b and 30b to perform interface reflection
cancellation. The manner in which electronic equalizers may be used
to perform interface refection cancellation is known to persons of
ordinary skill in the art.
Assuming, for exemplary purposes only, that each of the TXs 20a and
30a has a VCSEL or other laser diode that transmits at a data rate
of at least 10 Gb/s, and that each of the RXs 20b and 30b has a
photodiode (e.g., a P-I-N photodiode) that is capable of detecting
optical data signals at a data rate of at least 10 Gb/s, then the
aggregate data rate of the bi-directional optical link is at least
20 Gb/s. It should be noted, however, that the invention is not
limited with respect to the data rates of the TXs 20 and 30 or with
respect to the aggregate data rate of the link 1.
FIG. 2 illustrates a side cross-sectional view of an exemplary
embodiment of the optical coupling system 50 functionally
illustrated in FIG. 1. A laser diode (LD) 110 and a laser diode
driver IC 111 of the TX 20a depicted in FIG. 1 are shown in FIG. 2.
Also shown in FIG. 2 are the photodiode (PD) 120 and the RX IC 121
of the RX 30a depicted in FIG. 1. In accordance with this
exemplary, or illustrative, embodiment, the optical beam splitter
50a of the optical coupling system 50 includes an integrated
optical device comprising a substrate 50b, a plurality of optical
elements 50c-50e formed in or on the substrate 50b, an optical
coupling structure 50f disposed on an upper surface of the
substrate 50b, and a glass cover 50g disposed on an upper surface
of the optical coupling structure 50f.
In addition to the optical beam splitter 50a, the optical coupling
system 50 includes a lens assembly 130. As will be described below
6-13, the lens assembly 130 is a part of an optical transceiver
module (not shown). The lens assembly 130 mechanically couples the
ends of a plurality of optical fibers 10 to the optical beam
splitter 50a, although only one of the optical fibers 10 can be
seen in the cross-sectional side view of FIG. 2. Light being
coupled between the optical beam splitter 50a and the ends of the
optical fibers 10 passes through the glass cover 50g. An array of
refractive lenses 140 formed in the lens assembly 130 couple light
between the ends of the optical fibers 10 and the optical beam
splitter 50a. In the side view of FIG. 2, only the components
associated with only a single RX channel and a single TX channel
are shown. For example, only a single LD 110 of an array of LDs and
only a single PD 120 of an array of PDs are shown in FIG. 2.
Correspondingly, only a single refractive lens 140 of an array of
lenses and a single optical fiber 10 of an array of optical fibers
are shown in FIG. 2.
The manner in which the optical beam splitter 50a operates will now
be described with reference to FIG. 2. In the transmit direction,
the optical beam splitter 50a operates as follows. A main light
beam 141 produced by the LD 110 is collimated by a "big eye" ball
lens 112 and the collimated beam is directed onto the optical
element 50c, which, in accordance with this illustrative
embodiment, is a diffractive optical element. The diffractive
optical element 50c may be configured to perform the collimation
function, but using the big eye ball lenses 112 for this purpose
has certain advantages. In particular, using big eye ball lenses
for this purpose allows manufacturing tolerances to be relaxed. In
addition, using the big eye ball lenses 112 allows multiple small
arrays of VCSELs instead of a single large array of VCSELs to be
used for the LDs 110, which can provide significant cost
savings.
The diffractive optical element 50c splits the main light beam 141
into a first light portion L1, a second light portion L2, and a
third light portion L3. The first light portion L1 has about 10% to
30%, and typically about 20%, of the optical power of the main beam
141. The first light portion L1 corresponds to the optical data
signal that is ultimately coupled into the end of the optical fiber
10 for transmission over the link 1 (FIG. 1). The diffractive
optical element 50c also performs a beam tilting operation that
tilts the first light portion L1 at a predetermined angle relative
to an imaginary line (not shown) that is normal to the substrate
50b and that extends between the lower and upper surfaces of the
substrate 50b. In accordance with this illustrative embodiment, the
tilt angle ranges between about 5.degree. and 10.degree., and is
typically about 8.degree..
The tilted first light portion L1 passes out of the glass substrate
50b and into the optical coupling structure 50f. The optical
coupling structure 50f comprises a layer of replicated epoxy that
operates on the wavelength of light produced by the LD 110. The
optical coupling structure 50f is shaped to include facets 50f1,
50f2 and 50f3 that act as optically refractive and/or reflective
elements. Facet 50f1 operates as a refractive element that receives
the tilted first light portion L1 and tilts the beam such that the
beam is normal to the upper surface 50g'' of the glass cover 50g as
the beam passes through the glass cover 50g. In other words, the
facet 50f1 reverses the degree of tilt imparted on the first light
portion L1 by the diffractive optical element 50c. The glass cover
50g, in accordance with this illustrative embodiment, does not have
any optical power, and therefore, has no optical effect on light
passing through it. The purpose of the glass cover 50g is to
provide the optical beam splitter 50a with a clear flat surface
that enables it to be easily interfaced with the lens assembly 130,
as will be described below in more detail with reference to FIGS.
6-10
In the transmit direction, the refractive lens 140 of the lens
assembly 130 receives this light beam that passes through the glass
cover 50g and redirects it such that it is focused into the end of
optical fiber 10 for transmission over the link 1 (FIG. 1). As
indicated above, the first light portion L1 that is coupled into
the end of the optical fiber 10 typically has only about 20% of the
optical power of the main beam 141. Reducing the optical power of
the transmitted optical data signal in this manner ensures that the
optical data signal is within eye safety limits and yet has
sufficient optical power to prevent signal degradation from
detrimentally affecting the performance of the link 1 (FIG. 1). In
addition, using the optical beam splitter 50a to reduce the optical
power of the transmitted optical data signal obviates the need to
use other methods and devices to attenuate the signal to a level
that meets eye safety limits.
There are several advantages to using a relatively small tilt angle
for tilting the first light portion L1 relative to the angle of
incidence of the main beam on the diffractive optical element 50c.
An amount of tilt is needed to ensure that the optical beam
splitter 50a operates properly. For example, without some amount of
tilt, light traveling in the receive direction that is coupled from
the ends of the optical fibers 10 into the optical beam splitter
50a might be directed into the aperture (not shown) of the LD 110
efficiently, which could potentially destabilize the laser
operation. In addition, the relatively small tilt angle (e.g.,
8.degree.) allows the packaging of the optical beam splitter 50a to
be very compact and manufacturing tolerances to be relaxed. Large
tilt angles can be optically and mechanically difficult to achieve.
Splitters that provide large tilt angles sometimes encounter
polarization issues, and the materials that are used to make them
sometimes have different properties at different angles. Most
coatings are designed for perpendicular applications and
compensating for larger angles can be difficult. Also, coating
thickness tolerances are generally tighter for larger angle designs
than for smaller angle designs. Thus, the relatively small tilt
angle provided by the diffractive optical element 50c obviates many
of these manufacturing issues so that the optical beam splitter 50a
is easier to manufacture and can be manufactured to provide high
optical precision and efficiency.
The second light portion L2 has about 70% to 90%, and typically
about 80%, of the optical power of the main beam 141. The second
light portion L2 is the light portion that is ultimately absorbed
by the beam stop 70a (FIG. 1). In the illustrative embodiment shown
in FIG. 2, the second light portion L2 is reflected by the
diffractive optical element 50c into a gap that exists between the
LD 110 and the LD driver IC 111. The light that passes through this
gap is incident on a top surface 151 of a lead frame 150. The LD
110, the LD driver IC 111, the PD 120, and the RX IC 121 are
secured to the upper surface 151 of the lead frame 150 by a layer
of adhesive material 155, such as epoxy. In the embodiment of FIG.
2, the light passes through the adhesive material 155 and is
incident on the upper surface 151 of the leadframe 150. The light
is then reflected by the upper surface 151 of the leadframe 150
onto the substrate of the LD driver IC 111 and is absorbed thereby.
Thus, in accordance with this illustrative embodiment, the
substrate of the LD driver IC 111 functions as the beam stop 70a
shown in FIG. 1. The light may be reflected multiple times between
the upper surface 151 of the leadframe 150 and the substrate of the
laser driver IC 111 before the light is finally absorbed by the
substrate.
The invention is not limited to using any particular device or
material as the beam stop 70a. Essentially, any device or material
that functions as a light trap for light of the wavelength produced
by the LD 110 may be used for this purpose. For example, the
adhesive material 155 may be an epoxy that is absorptive to light
of the wavelength that is produced by the LD 110. As will be
understood by persons skilled in the art, other types of light
traps may be used for this purpose. By absorbing the second light
portion L2 in this manner, the possibility of stray light
detrimentally impacting the operations of the optical transceiver
module 20 (FIG. 1) is eliminated.
The optical coupling system 50 also splits off a small light
portion L3 of the main beam 141 for use in optical feedback
monitoring. In accordance with this illustrative embodiment, the LD
driver IC 111 includes an array of monitor photodiodes 161, only
one of which can be seen in the side cross-sectional view of FIG.
2, for monitoring the optical output power levels of the LDs 110.
The third light portion L3 typically has about 1% to 2% of the
optical power of the main beam 141, which is a sufficient amount
for optical feedback monitoring. The diffractive optical element
50c tilts the third light portion L3 at a relatively small angle
(e.g., about 15.degree.) relative to an imaginary line that is
normal to lower surface of the substrate 50b and that extends
between the lower and upper surfaces of the substrate 50b. The
facets 50f2 and 50f3 act as a minor that internally reflects the
third light portion L3 to fold the third light portion L3 and
direct it back down through the substrate 50b and onto the monitor
photodiode 161.
In the receive direction, light L4 passing out of the end of the
optical fiber 10 is directed by the refractive lens 140 onto the
facet 50f1. The facet 50f1 tilts the incoming beam by a
predetermined tilt angle and directs it onto the diffractive
optical element 50c. The predetermined tilt angle is about
8.degree. relative to an imaginary line that is normal to the upper
surface of the substrate 50b and that extends between the upper and
lower surfaces of the substrate 50b. Although a small portion of
the incoming beam L4 may pass through the diffractive optical
element 50c and fall on the LD 110, the tilt angle and the
diffraction created by the diffractive optical element 50c ensure
that this small portion of light does not fall on the aperture of
the LD 110, and therefore will not have a detrimental impact on the
performance of the LD 110. Approximately 80% of the incoming light
L4 that passes out of the end of the optical fiber 10 is directed
by the diffractive optical element 50c onto the optical element
50d, which is a reflective optical element. The reflective optical
element 50d is a minor that is created either by depositing a
reflective layer of metal on the glass substrate 50b to form a
mirror or by depositing layers of dielectric material on the glass
substrate 50b to form a dielectric minor.
The light that is incident on the reflective optical element 50d is
directed by the reflective optical element 50d onto a refractive
lens 50e, which focuses the light onto the PD 120. Like the facets
50f1-50f3 of the optical coupling structure 50f, the refractive
lens 50e is formed of replicated epoxy, as will be described below
in detail with reference to FIGS. 3A-5B. Although an array of the
refractive lenses 50e are formed on the glass substrate 50a, only
one of the refractive lenses 50e can be seen in the cross-sectional
side view of FIG. 2. Alternatively, lens 50e may also be a
diffractive lens that focuses the light onto the PD 120. This
diffractive lens may also be formed of replicated epoxy.
The manner in which the optical beam splitter 50a shown in FIG. 2
is fabricated will now be described with reference to FIGS. 3A-5B.
FIGS. 3A and 3B illustrate bottom and top perspective views,
respectively, of a glass wafer 200 having arrays of the optical
elements 50c and 50d, respectively, formed on the bottom and top
surfaces 200a and 200b, respectively. With reference to FIG. 3A,
the optical elements 50c correspond to the diffractive optical
elements 50c described above with reference to FIG. 2. Although the
wafer 200 is shown in FIG. 3A as having only about twenty of the
diffractive optical elements 50c arrayed on the bottom surface 200a
thereof, there are typically hundreds or thousands of the
diffractive optical elements 50c formed on the bottom surface 200a
of the wafer 200. The diffractive optical elements 50c have
identical diffractive patterns formed therein. The diffractive
patterns may be formed in a variety of ways, such as by etching,
chemical vapor deposition (CVD), sputtering, etc. Each of the
diffractive patterns is essentially a holographic pattern made up
of a series of depth variations in the bottom surface 200a, which
can be formed by either removing portions in selected areas of the
bottom surface 200a to create the depth variations or by adding
material (e.g., metal) to selected areas of the bottom surface 200a
to create the depth variations. For example, a wet or dry etching
technique may be used to remove selected portions of the bottom
surface 200a, whereas CVD or sputtering may be used to selective
add material to the bottom surface 200a of the wafer 200.
With reference to FIG. 3B, the optical elements 50d correspond to
the refractive optical elements 50d shown in FIG. 2. The refractive
optical elements 50d are minors formed either by depositing a
reflective coating (e.g., metal) on the top surface 200b or by
depositing layers of a dielectric material on the top surface 200b
to create a dielectric minor. Although the wafer 200 is shown in
FIG. 3B as having only about twenty of the refractive optical
elements 50d arrayed on the top surface 200b of the wafer 200,
there are typically hundreds or thousands of the refractive optical
elements 50d formed on the top surface 200b of the wafer 200.
FIGS. 4A and 4B illustrate the bottom and top surfaces 200a and
200b of the wafer 200 after an epoxy replication process has been
used to form the refractive optical elements 50e on the bottom
surface 200a of the wafer 200 and to form the optical coupling
structure 50f on the top surface 200b of the wafer 200. The epoxy
replication process is a known process that uses a master, or mold,
to create a pattern, or replica that is transferred to another
surface. During the epoxy replication process, a master having a
shape corresponding to the shape of the array of optical elements
50e is filled with a liquid epoxy. The epoxy is then cured to
transfer the shape of the master into the cured epoxy, thereby
forming an epoxy replica 210. The master is separated from the
epoxy replica 210, leaving the replica 210 disposed on the bottom
surface 200a of the wafer 200 as shown in FIG. 4A. The epoxy
replica 210 covers the array of diffractive optical elements 50c,
but is transparent to the wavelength of light that is produced by
the LD 110. Similarly, on the top surface 200b of the wafer 200,
the epoxy replication process is performed during which a master
having a shape corresponding to the shape of the optical coupling
structure 50f is filled with a liquid epoxy. The epoxy is then
cured to transfer the shape of the master into the cured epoxy,
thereby forming an epoxy replica 220. The master is then separated
from the replica 220, leaving the replica 220 disposed on the top
surface 200b of the wafer 200 as shown in FIG. 4B.
FIGS. 5A and 5B illustrate bottom and top perspective views,
respectively, of the optical beam splitter 50a shown in FIG. 2.
After the epoxy replicas 210 and 220 have been created in the
manner described above with reference to FIGS. 4A and 4B, a
singulation process is performed during which the wafer 200 is
sawed to separate the optical beam splitters 50a from one another.
As indicated above, hundreds or thousands of the optical beam
splitters 50a may be formed on a single wafer 200. Therefore, the
manufacturing yield for the optical beam splitter 50a is very high
and the splitters 50a are manufactured with very high precision.
The optical beam splitter 60a shown in FIG. 2 is made by the same
process and has a configuration that is identical to the
configuration of the optical beam splitter 50a described above with
reference to FIGS. 2-5A.
FIG. 6 illustrates a top perspective view of an optical transceiver
module 300 having an optical beam splitter holder 310 coupled
therewith that holds the optical beam splitter 50a shown in FIG. 2.
In FIG. 6, the only component of the optical beam splitter 50a that
is visible is the glass cover 50g. FIG. 7 illustrates a
cross-sectional perspective view of the optical beam splitter
holder shown in FIG. 6 with the optical beam splitter 50a held
therein. With reference to FIG. 6, the optical transceiver module
300 includes a printed circuit board (PCB) 320, the leadframe 150
shown in FIG. 2 mounted on an upper surface of the PCB 320, and
although not visible in FIG. 6, the LD 110, the LD driver IC 111,
the PD 120 and the RX IC 121 shown in FIG. 2 mounted on the
leadframe 150. The LD 110, the LD driver IC 111, the PD 120 and the
RX IC 121 are blocked from view in FIG. 6 by the optical beam
splitter holder 310. The PCB 320 has a land grid array (LGA) 330
formed on the lower surface thereof for electrically coupling the
LD 110, the LD driver IC 111, the PD 120, and the RX IC 121 to
electrical contacts (not shown) of a motherboard (not shown) on
which the optical transceiver module 300 may be mounted.
Holes 340a and 340b formed in opposite ends of the optical beam
splitter holder 310 are shaped and sized to receive pins (not
shown) located on the lens assembly 130 (FIG. 2), as will be
described below with reference to FIG. 9B. Holes 350a-350d formed
in the optical beam splitter holder 310 are used to align the
optical beam splitter holder 310 with the optical transceiver
module 300 when the optical beam splitter holder 310 is being
mounted on the optical transceiver module 300. Retention features
360a and 360b formed on opposite ends of the optical beam splitter
holder 310 are used to mechanically couple the holder 310 to an
optical connector (not shown), as will be described below in more
detail with reference to FIGS. 8-9B.
With reference to the cross-sectional perspective view of FIG. 7,
the manner in which the optical beam splitter 50a is held within
the optical beam splitter holder 310 is demonstrated. The lower
surface 50g'' of the glass cover 50g is mechanically coupled with
the optical coupling structure 50f of the optical beam splitter
50a. The optical beam splitter holder 310 has an opening 370a
formed therein for receiving and mating with the optical beam
splitter 50a. Portions of the lower surface 50g'' of the glass
cover 50g rest on a recessed ledge 370b formed within the opening
370a. The portions of the lower surface 50g'' of the glass cover
50g that rest on the recessed ledge 370b may be secured to the
recessed ledge 370b by an adhesive material (not shown) to
mechanically couple the optical beam splitter 50a to the optical
beam splitter holder 310. The upper surface 50g'' of the glass
cover 50g is flush with the upper surface 380 of the optical beam
splitter holder 310. These features eliminates gaps and enable the
upper surface 50g'' of the glass cover 50g to be easily cleaned by
wiping the upper surface 50g'' with a finger or a piece of cloth or
other material. This is not a feature of optical transceiver
modules that are currently available in the market. Most currently
available optical transceiver modules have gaps in them that trap
contaminates. Efforts to remove these contaminates are sometimes
made by blowing air into the gaps, but oftentimes such efforts are
unsuccessful. Of course, contaminates existing in the optical
pathways can detrimentally affect the performance of the optical
transceiver modules. Thus, providing a beam splitter configuration
that enables the glass cover 50g to be wiped clean to remove
contaminates is highly advantageous.
FIG. 8 illustrates a perspective view of the optical transceiver
module 300 shown in FIG. 6 having an optical connector 400 coupled
to it, which, in turn, has the lens assembly 130 shown in FIG. 2
coupled to it. FIGS. 9A and 9B illustrate top and bottom
perspective views, respectively, of the optical connector 400 shown
in FIG. 8 having the lens assembly 130 shown in FIG. 2 coupled to
it. The lens assembly 130 is mechanically coupled to the optical
connector 400 by snap brackets 410a and 410b formed on the optical
connector 400 that snap into indentations 130a and 130b,
respectively, formed in the lens assembly 130. The lens assembly
130 has pins 130c and 130d located thereon that mate with the holes
340a and 340b (FIGS. 6 and 7), respectively, formed in the optical
beam splitter holder 310. This mechanical coupling arrangement
allows some freedom of movement of the lens assembly 130 relative
to the optical connector 400. In other words, the lens assembly 130
"floats" relative to the optical connector 400, which ensures
precision alignment of the lens assembly 130 with the optical beam
splitter holder 310 when the optical connector 400 is coupled with
the holder 310, as will be described below in more detail.
The optical connector 400 has latches 420a and 420b formed on
opposite ends thereof that mechanically couple with the retention
features 360a and 360b, respectively, formed on the optical beam
splitter holder 310. Although the optical connector 400 is
typically made of a molded plastic material that provides the
connector 400 with a generally rigid structure, the latches 420a
and 420b flex to a limited degree when engaged with the retention
features 360a and 360b to provide a spring force that is exerted on
the optical connector 400 to firmly press portions 440 of the
optical connector 400 against the optical beam splitter holder 310.
As the optical connector 400 and the optical beam splitter holder
310 are pressed together in this manner and interlocked, the pins
130c and 130d formed on the lens assembly 130 mate with the holes
340a and 340b, respectively, formed in the optical beam splitter
holder 310 to precisely align the lens assembly 130 with the
optical beam splitter holder 310.
An optical fiber ribbon cable 450 comprising a plurality of optical
fibers 460 is coupled on an end thereof with the lens assembly 130
via a cover 470 that presses the ends of the fibers 460 against
V-shaped grooves (not shown) formed in the lens assembly 130, as
will be described below in more detail with reference to FIG. 10.
The refractive lenses 140 described above with reference to FIG. 2
are covered by a piece of protective tape 480 that protects the
refractive lenses 140 from contaminants.
FIG. 10 illustrates a cross-sectional side view of the optical
transceiver module 300 shown in FIG. 8 coupled with the optical
connector 400, which, in turn, is coupled with the lens assembly
130. In FIG. 10, the manner in which light is coupled between the
ends of the optical fibers 460 and the optical beam splitter 50a is
demonstrated. Like reference numerals in FIGS. 8-10 represent like
components. With reference to FIG. 10, the refractive lenses 140
formed on the lens assembly 130 are 45.degree. mirrors. In the
transmit direction, light produced by the LDs 110 is guided by the
optical beam splitter 50a in the manner described above with
reference to FIG. 2 and is directed be the splitter 50a through the
glass cover 50g. The light directed through the glass cover 50g is
incident on the refractive lenses 140 and is reflected at
45.degree. angles onto the ends of respective optical fibers 460.
In the receive direction, light passing out of the ends of the
optical fibers 460 is incident on the refractive lenses 140 and is
reflected at 45.degree. angles such that the light is directed
through the glass cover 50g and is incident on the optical coupling
structure 50f. The received light is then guided in the manner
described above with reference to FIG. 2.
It should be noted that the invention has been described with
reference to a few illustrative, or exemplary, embodiments for the
purposes of demonstrating the principles and concepts of the
invention. It will understood by persons skilled in the art that
the invention is not limited to the embodiments described herein
and that many modifications may be made to the embodiments
described herein without deviating from the invention. For example,
the invention is not limited to any particular configuration for
the optical beam splitter 50a or to any particular percentages for
the optical power of the light portions that are split off from the
main beam. Also, which the optical beam splitter 50a has been
described as having a bidirectional configuration for
implementation in a bidirectional link, the principles and concepts
described herein are equally applicable to unidirectional
applications.
* * * * *